Acid phosphatase in human prostate (PAP) is antigenically unique secretory enzyme. Therefore, a few immunologic assay methods (RIA, EIA, IF) have been developed for this enzyme to explore the correlation between serum PAP levels or the PAP-secreting metastatic prostatic cancer cells in the biopsy materials with the clinical progression of prostatic carcinoma. In order to elucidate the molecular basis of antigenic specificity of PAP, monoclonal anti-PAP antibodies were prepared by hybridoma technique and these antibodies were tested against PAP, subunit polypeptides, and PAP-derived peptides. As a complementary approach, antibodies to acid phosphatase derived from the lysosomal fraction of human fibroblasts were prepared, and the antigenic difference between the PAP and other lysosomal acid phosphatases have been investigated.